In the automatic grading of agricultural produce such as tomatoes and apples for example, the articles of produce are moved in alignment along a conveyor and past an inspection station where they are automatically graded and then sorted according to some desired characteristic of the produce. Quite commonly the basis for grading is the color of the articles and the grading decision is based on the comparison of two or more electrical signals which are functions of respective color components of light which are reflected from the articles onto photoelectric detectors. This general method of grading agricultural products is well known and need not be further described.
In sorting articles of produce according to color, the grading classification will include a given range of color and intensity variations. The optical system associated with the photodetectors of the color grader must be good enough that the variation in color and intensity of light reflected from a single article and transmitted through the optical system is well within the range of variations established for grading the particular produce.
In a moving conveyor type of produce grader the high speed of movement of the articles through the field of view of the detectors, the variation in sizes of the articles, the vibration and sometimes rolling of the articles on the conveyor all cause the viewing distance and viewing angle to vary as the articles pass through the field of view. These variable factors have in the past caused significant variations in the color and intensity of light transmitted by the optical system and received by the color detectors. The result has been that the detectors have produced inconsistent signals, thus causing the electronic grading equipment to compare color signals which were not truly representative of the color components of the article being viewed at a given instant of time.
One type of optical system used in the past to direct reflected light onto a plurality of spaced color detectors was comprised of an objective lens, a field stop, and a field lens, wherein the field lens was intended to present equally to two or three spaced apart color detectors the light from the field of view which enters the objective lens from objects in the field of view. That is, the field lens focused the image of the clear aperture of the objective lens onto the detectors. However, if the article being viewed were not exactly in focus at the field stop during any part of its travel through the field of view, the possibility existed that not all the detectors would be equally illuminated. Additionally, as the object moved across the field of view of the lens system, the angle of the light reflected back to the optic system constantly changed so that it was possible that all the detectors did not receive the same amount of light even if the object were in focus. The nonuniform illumination of the detectors gave rise to the generation of erroneous color component signals and resulted in erroneous grading.